Doxyfile: imitate gcc for better libavutil/attributes.h documentation

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/Doxyfile

@@ -1359,6 +1359,8 @@ PREDEFINED             = "__attribute__(x)=" \
                          "DECLARE_ALIGNED(a,t,n)=t n" \
                          "offsetof(x,y)=0x42" \
                          av_alloc_size \
+                         AV_GCC_VERSION_AT_LEAST(x,y)=1 \
+                         __GNUC__=1 \
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.